Electrosurgery is the application of a radio frequency electrical energy to a surgical site on a human or animal for tissue cutting, coagulation, or a blend thereof. In monopolar mode the radio frequency current that is generated by an ESU is applied to tissue from an active electrode held by the surgeon, and is collected from a dispersive electrode attached to the patient. A small contact area of the active electrode causes a high current density so that a spark enters the tissue at the surgical site. This spark causes intense localized heating, eschar, fulguration and other effects, to achieve the cutting and/or coagulation. The dispersive electrode collects the energy returning it to the ESU to complete an electrical circuit. The dispersive electrode is of a significant size so that the energy density collected thereby is low enough to avoid any surgical or heating effect that would burn.
A burn will develop if the power delivered to the tissue and after its passage through the body results in a high energy density at the exit so that localized tissue heating occurs. This situation happens when the energy is allowed to leave a patient's body at a location other than the dispersive electrode. Such a condition is called leakage. A burn from leakage can be quite severe as the patient is anesthetized and will not react thereto. The burn area is frequently covered so the doctor or surgical attendants will not see it until it is too late to take corrective action.
Another potential path for leakage burns is to the surgeon through contact with the active electrode or the conductors which supply the radio frequency, high voltage electrosurgical energy. Leakage in that circumstance may harm or burn the surgeon or one of the surgical attendants in contact with the active electrode or its supply conductor and a ground. It is for this reason that leakage or alternate path energy flow in electrosurgery are of considerable concern and efforts are made to monitor and control leakage.
The early electrosurgical units (ESU) were of a ground referenced design. Being ground referenced, the return for the ESU and the dispersive electrode were both connected to earth or ground. The ground referenced arrangement was satisfactory provided that no other point on the patient was grounded. When a monitoring electrode, i.e. EKG, was used during the electrosurgical procedure, and the monitoring electrode was referenced to ground, some portion of the electrosurgical energy could flow to ground through the monitoring electrode, instead of the preferred path back through the dispersive electrode. Since monitoring electrodes usually have small contact area, the current density at their contact may be sufficient to develop enough energy density to result in a burn. An even worse condition occurs if the electrosurgical generator connection to the dispersive electrode is accidentally separated. Thus, with no direct energy path back to the ESU, all of the power travels through any alternate grounded paths, such as through the monitoring electrodes, the surgeon and/or the surgical table. Severe burns are a possible result.
In an effort to reduce the risks associated with the ground referenced ESUs, the power output circuit of the ESU was isolated from any other ground. Output isolated ESUs were a significant step in reducing the risks associated with alternate path burns, because the electrosurgical energy exiting the patient was more likely to flow through the dispersive electrode to complete the circuit and not through any other ground referenced points when returning to the ESU. If the generator connection to the dispersive electrode became disconnected, a significant portion of the electrosurgical energy flow from the ESU would stop.
Although isolated output ESUs was an improvement over the previous ground referenced units, a problem remained because the isolation from ground was not always perfect. At the relatively high frequencies of electrosurgical current, e.g., 500 kilohertz to 1 megahertz, stray capacitance to ground allows another ground referenced path. Furthermore, the amount of stray capacitance required to create this other significant path for ground referenced energy flow is not great. Although alternate paths of energy flow are less than those flowing if the ESU was ground referenced, a potential exists for significant patient and alternate path burns.
An improvement to help minimize alternate paths for energy in isolated electrosurgical generators includes the use of a differential transformer in the output circuit, as shown in U.S. Pat. No. 4,437,464. The electrosurgical energy supplied to the active electrode flows through a winding on a transformer core, and the energy from the dispersive electrode flows through a winding wound opposite to the direction of the winding for the active energy flow on that core. Normally the energy passing through the two windings are equal and of opposite direction, as would be the case when there is no alternate path. Thus, the counteracting fluxes therefrom cancel each other. The transformer core presents very little loss or impedance to the flow of electrosurgical energy.
If a significant alternate path exists, the imbalance created thereby results in a flux in the core of the differential transformer causing a measurable loss that increases the impedance and reduces the amount of energy flowing to the active electrode. Thus, the current flow through the active electrode to the patient is automatically inhibited and therefore reduced, thereby causing a commensurate decrease in the alternate path leakage flow. Although this approach reduces leakage, it may not be sufficient to reduce the leakage below a maximum acceptable safe energy level, for example one hundred fifty milliamps.
Another improvement, which provides an alarm or terminates the delivery of electrosurgical power under conditions of excessive leakage with an isolated ESU, is disclosed in U.S. Pat. No. 3,683,923. A third or sensing winding on the differential transformer responds to the imbalance in the flow of energy through the active winding and the return winding. The third winding, upon sensing a sufficient imbalance between the energy flow, triggers an alarm circuit for the operator. A relay may simultaneously or alternatively be activated to terminate the flow of energy to the tissue. The operator may take corrective action such as reducing the power level or attempting to eliminate the problem causing leakage, as well as reactivating the ESU.
U.S. Pat. No. 4,094,320, assigned to the owner of the present invention, has a compensating means for varying the threshold at which the leakage current detected will control the output signal of the generator. The sensitivity of the threshold is thereby regulated. U.S. Pat. No. 4,188,927, assigned to the assignee as the present invention, has a leakage threshold varied in accord with the mode selected so that power output is lower with the desiccation mode than with a mode that permits arcing. A further approach is the use of the signal from the third winding as input for an automatic feedback loop that controls the energy output from the electrosurgical generator to the patient. Such control responds to the leakage measured, as a function of the difference between active and return energy flow, by reducing the output smoothly. U.S. Pat. No. 5,152,762 discloses such a circuit designed to apply the past technology for sensing the leakage to circuitry including a feedback control having a loop to regulate the ESU output. Imbalance is sensed in an isolation transformer winding responsive to the difference in energy flow between the active and return electrodes. The signal generated is considered with an accepted maximum amount and then the requested output to insure that the ESU output is regulated. U.S. Pat. No. 4,658,819, assigned to the assignee of the invention herein, has a circuit that decreases the output power in accord with the square of the increase of the impedance.
The problem of the transient conditions including varying loadings or sparks or arcing during the initiation or termination of the electrosurgical effects remains. Specifically, situations wherein the active electrode is not in electrical contact with the patient's tissue such that the energy transmitted to the tip of the electrode must be sufficient to complete the open circuit without causing leakage. Those transient conditions require accelerated handling of the imbalance measured in the transformer. The leakage circuitry must be able to not only take into account the activated mode of the generator but also change the sampling rate of the signal as the leakage becomes more critical.
Against this background and with an appreciation of the problem of transient conditions, further significant improvements and advancements in the control of leakage currents, particularly during initiation and termination, to account for open circuit conditions, are required. Described herein are an instantaneous leakage control and a method of its use that is not found in the literature or practiced in the field. The literature is of interest for its teachings of the knowledge of skilled artisans at the time of this invention of a leakage control and a method use thereof.